Cation energy storage device and methods

ABSTRACT

An energy storage composition can be used as a new Na-ion battery cathode material. The energy storage composition with an alluaudite phase of A x T y (PO4) z , Na x T y (PO4) z , Na 1.702 Fe 3 (PO4) 3  and Na 0.872 Fe 3 (PO4) 3 , is described including the hydrothermal synthesis, crystal structure, and electrochemical properties. After ball milling and carbon coating, the compositions described herein demonstrate a reversible capacity, such as about 140.7 mAh/g. In addition these compositions exhibit good cycling performance (93% of the initial capacity is retained after 50 cycles) and excellent rate capability. These alluaudite compounds represent a new cathode material for large-scale battery applications that are earth-abundant and sustainable.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is being filed on 7 Apr. 2016, as a PCTInternational application and claims priority to U.S. Provisional PatentApplication No. 62/144,021, filed Apr. 7, 2015, and entitled “Sodium-IonEnergy Storage Device,” the disclosure of which is hereby incorporatedby reference in its entirety.

TECHNICAL FIELD

The disclosure relates generally to energy storage compositions. Morespecifically, energy storage compositions that can be used as cathodematerials. The disclosure also relates to methods of making energystorage compositions using hydrothermal processes. The disclosure alsodescribes methods of using energy storage compositions.

INTRODUCTION

The demand for efficiency improvements in energy storage systems isdriving the development of batteries with higher energy density,increased depth of discharge, longer life cycles, and lighter, flexibleform factors. Most current research efforts are directed towardsLithium-ion (Li-ion) batteries because of their inherent higher energydensity compared to other types of rechargeable battery chemistries, andtheir negligible memory effect after numerous charge-discharge cycles.Thus, for approximately the past twenty years, significant resourceshave been directed toward improving the electrochemical performance ofactive electrode materials, developing safer electrodes andelectrolytes, and lowering the manufacturing cost of Li-ion batteries.

However, Li-ion batteries are designed to meet specific applicationrequirements and tradeoffs are often made between various parameters,such as high energy density vs. high power, charge-discharge rate vs.capacity and cycle life, safety vs. cost, etc. These tradeoffs becomenecessary, primarily due to the limitations imposed by theelectrochemical properties of the active materials, electrolytes, andseparators as well as battery manufacturing methods.

Lithium-ion batteries are manufactured in various shapes and sizes andare widely used in various types of portable electronic devices,including medical devices and are also being considered for use inelectric vehicles, solar power systems, smart electricity grids, andelectric tools. Current Li-ion battery technology, however, is limitedin terms of energy capacity, charging speed, and manufacturing cost.Based on Department of Energy (DOE) reports, ten years of effort, andbillions in spending on Li-ion battery development, the manufacturingcost of Li-ion batteries has not decreased significantly and is stillthree to six times higher than the DOE target ($700/kWh-current vs.$150/kWh-target).

The concern about the overuse of fossil fuels has stimulated research onsustainable approaches to meet our energy demands. One solution is tomake better use of renewable energy, such as solar, wind, and wavepower. However, these sources of energy vary in time and space, thusstimulating a demand to develop efficient and reliable energy storagesystems. The Li-ion battery dominates the portable electronic marketbecause of its high energy density, flexible design, and long servicelife. However, the increasing costs and potential geopoliticalconstraints on lithium reserves make Li-ion batteries unsuitable forlarge-scale energy storage applications.

Furthermore, performance of Li-ion batteries has not improved asexpected, especially for scalable manufacturing platforms. A keycontributor to the price stagnation and performance plateau is continuedreliance on the same traditional battery manufacturing technology usingroll-to-roll foil lamination that was developed over twenty years ago.Another contributing factor is the synthesis of the powder based activeelectrode material, which constitutes 40-50% of the battery cost. Thus,a new battery design and manufacturing paradigm is required to addresscost issues. Also, graphite anode based Li-ion battery technology islimited in terms of energy capacity, charging speed, and safety. Becauseof limited anode capacity, batteries require charging more often.Competitive anode solutions have not overcome fundamental challenges,resulting in limited calendar life as well as slow charging.

Previous solid state synthesis provided a cathode material under properconditions that will improve battery performance, as elevatedtemperature annealing causes the cathode material to crystallize.However, elevated temperature annealing increases the cost of cathodemanufacturing.

Lithium-ion, sodium-ion (Na-ion), or Li—Na multi-ion secondary batteriesare known to have high energy densities. For sufficient power, thickcathodes are employed in these batteries. Over the years, these cathodeswere fabricated by a series of complex and expensive techniques. Suchtechniques include forming nanoscale powders of active cathode material,mixing the active powder with an inert organic binder dissolved inappropriate solvent to form a slurry. Various slurry coating techniquesare used to form the thick film of the cathode on a metallic substratefollowed by calendaring and drying processes to fully stabilize and formthe cathode. Another cathode formation scheme involves mixing thecathode material (as a nano-particle powder) with the binder powder andpressing the mixture to form a pellet or a plate cathode, followed bydrying. The inert binder content in these cathodes could be as high as30% and unnecessarily lowers the power density of batteries containingthem.

However, traditional methods of making the powder and deploying thepowder to make the film are cumbersome, and more streamlined methods areneeded to enable wider adoption of energy compositions that are notsolely lithium based.

Vacuum deposition techniques, sputtering, chemical vapor deposition, andpulse laser deposition, have been adopted to grow organic binder freeinorganic cathode films. These processes are slow and expensive, and thegrown films are thin, less than 5 μm. The latter are therefore suitableonly for microbatteries. Adopting these processes to grow thicker filmon a large area would not be economical, because the capital equipmentcost or/and operation cost will be too high.

Binder-free cathode films have also been grown by electrostatic spraydeposition. Here, the solution consisting of lithium, salt and metalsalt dissolved in ethanol or ethanol and butyl carbitol mixture ispumped to a metallic capillary nozzle. A DC voltage above 5 kV appliedbetween the metallic nozzle and the heated metallic substrate generatesa mist by electrohydrodynamic force. The electrostatic force then movesthe mist to the hot substrate at temperature between 240° C. to 450° C.where the film gets deposited by pyrolysis of the mist. About 1 to 5 mthick film could be deposited by this technique per hour, therefore verysuitable for microbatteries.

Several alluaudite compounds have been investigated for batteryapplications, including: Li_(x)Na_(2−x)FeMn₂(PO₄)₃NaMnFe₂(PO₄)₃,Li_(0.5)Na_(0.5)MnFe₂(PO₄)₃ and Li_(0.75)Na_(0.25)MnFe₂(PO₄)₃,Li_(0.47)Na_(0.2)FePO₄, and Li_(0.78)Na_(0.22)MnPO₄. When used ascathode materials in Li-ion batteries, both Li_(0.47)Na_(0.2)FePO₄ (140mAh/g) and Li_(0.78)Na_(0.22)MnPO₄ (135 mAh/g) exhibit relatively highcapacity and good cycling performance. However, when used as cathodematerials in Na-ion batteries, these materials exhibit poorelectrochemical properties including low capacity and significantpolarization under load.

The alluaudite compositions described herein improve the performance ofalluaudite materials. In particular the ability to improve theperformance of energy storage compositions and cathode materials inbatteries. The disclosure also provides an improved method ofmanufacture to more efficiently make nano-scale materials (i.e. 200 nm)with improved electrochemical characteristics and reduced raw materialcosts.

Na-ion batteries are an excellent candidate to overcome the detrimentalaspects of Lithium-ion batteries. For example Na-ion batteries should beless expensive than Li-ion batteries because the raw materials for aNa-ion battery are far more abundant than that of a Li-ion battery.Moreover, Na-ion compounds exist in a variety of novel intercalationstructures that are not found as Li-ion compounds.

It is understood that both the foregoing general description and thefollowing detailed description are exemplary and explanatory and areintended to provide further explanation of the novel aspects of thetechnology as claimed.

Cation Energy Storage Device and Methods

This disclosure is directed to achieving the aforementioned unmet needswith a cation energy storage composition that is made from sustainableelements and is characterized by a capacity that is similar totraditional Lithium-ion (Li-ion) batteries.

Sodium-ion (Na-ion) batteries hold promise as an enabling technology forlarge-scale energy storage that is safer, less expensive, and have amuch smaller environmental impact than their equivalent Li-ionbatteries. As described herein, an alluaudite phase ofA_(x)T_(y)(PO4)_(z); Na_(x)T_(y)(PO4)_(z); Na_(1.702)Fe₃(PO4)₃ andNa_(0.872)Fe₃(PO4)₃, can be utilized as new Na-ion battery cathodematerials. These alluaudite compositions are described including thehydrothermal synthesis, crystal structure, and electrochemicalproperties. In at least some of the alluaudite compositions, A isselected from a group consisting of Li, Na, Mg, Ca and combinationsthereof. Additionally, T is selected from a group consisting of Fe, Mn,Co, Ni, Al, Sn and combinations thereof. After ball milling and carboncoating, the compositions described herein demonstrate a reversiblecapacity, such as about 140.7 mAh/g. In addition, these compositionsexhibit good cycling performance (93% of the initial capacity isretained after 50 cycles) and excellent rate capability. Thesealluaudite compounds represent a new cathode material for large-scalebattery applications that are earth-abundant and sustainable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration comparing lithium and sodium.

FIG. 2 is an illustration of sodium and lithium batteries.

FIG. 3 depicts phosphate based compounds with thermal and chemicalstability.

FIG. 4 is an illustration of an (a) alluaudite structure and (b) acharge/discharge reaction with the chemical formula Na_(x)Fe₃(PO₄)₃.

FIG. 5 is an illustration of one example of hydrothermal synthesis ofNa_(x)Fe₃(PO₄)₃.

FIG. 6a is a graphical representation of X-ray Diffraction (XRD) patternand Rietveld refinement of Na_(1.702)Fe₃(PO₄)₃

FIG. 6b is an illustration of the structure of alluaudite, where Fe²⁺and Fe³⁺ ions are represented octahedra and PO⁴⁻ ions are represented astetrahedral.

FIG. 7 depicts SEM micrographs showing the morphology and particle sizeof (a) as-synthesized composition, (b) annealed composition, and (c)carbon-coated composition and (d) TEM image of an as-synthesizednanoplate of the composition.

FIG. 8 is an illustration of larger contact between electrode andelectrolyte.

FIG. 9 is an illustration of one embodiment of the carbon coatingprocess.

FIG. 10a is a graphical representation of alluaudite sample as cathodematerial.

FIG. 10b is a graphical representation of cyclic voltammetry of cathodematerial.

FIG. 11 is a graphical representation of discharge capacity of energystorage composition over 30 cycles.

FIG. 12 is an illustration of one example embodiment of a ball millingprocess.

FIG. 13 is SEM image of ball-milled energy storage composition.

FIG. 14 is a graphical representation of one example embodiment of thedischarge capacity of a ball-milled energy storage composition.

FIG. 15 is a graphical representation of a ball-milled energy storagecomposition discharge capacity over 30 cycles.

FIG. 16 is a graphical representation of the discharge capacitycomparing one example embodiment of carbon coated ball-milled energystorage composition and non-carbon coated ball-milled energy storagecomposition.

FIG. 17 is a graphical representation of the discharge capacity over 30cycles comparing one example embodiment of carbon coated ball-milledenergy storage composition and non-carbon coated ball-milled energystorage composition.

FIG. 18 describes a carbon coated energy storage composition (a) beforeand (b) after ball-milling.

FIG. 19 is a graphical representation of the discharge capacity over 30cycles comparing one example embodiment of carbon coated ball-milledenergy storage composition and carbon coated non ball-milled energystorage composition.

FIG. 20 is a graphical representation of rate capability of one exampleembodiment of an energy storage composition.

FIG. 21 is a graphical comparison of one example embodiment of an energystorage composition compared with other cathode materials.

DEFINITIONS

As used herein, the term “energy storage composition” defines a cationcomposition for energy storage.

As used herein, the term “cathode material” defines a cation compositionused as a precursor-material or material for the making of a cathode.

As used herein, the term “about” defines 10% variation of the valuedefined.

As used herein, the term composition describes an aggregate chemicalsubstance formed from the interaction of at least two chemicalcompounds.

DETAILED DESCRIPTION

Sodium-ion (Na-ion) batteries hold promise as an enabling technology forlarge-scale energy storage that is safer, less expensive, and lower interms of environmental impact than their equivalent Lithium-ion (Li-ion)batteries. FIG. 1 shows a comparison of Li and Na. Specifically, FIG. 1,compares the atomic weight, cation radius, standard electrode potential,terrestrial reserve, cost and minerals. It is important to note thatthere are many more known sodium compounds than lithium compounds. Thatmeans there are more choices for electrode and electrolyte materials inthe Na system. FIG. 2 describes a Na-ion and Li-ion battery arrangement.As shown, current collector is metal foil, in this case Aluminum (Al) isused to ensure maximum current efficiency. The main purpose for theelectrode materials is that they should be able to achieve insertion andextraction of Na+. Thus, in these particular embodiments, channeledstructure or layered structures are preferred. One of ordinary skill inthe art should appreciate that the Na-ion battery is similar to Li-ionbattery, with several differences. One difference is the charge carrieris Na+ instead of Li+. Also, graphite cannot be used as the anodematerial in a Na-ion battery and the cathode materials are alsodifferent.

FIG. 3 illustrates the various electrode materials. In particular,different materials that have been tested for Na-ion battery. Thesematerials that have low voltage are used for anode material. Forexample, Hard C has 300 mAh/g, which is a very good electrode material.Phosphate materials are also good electrode materials because of theirthermal and chemical stability.

Energy Storage Composition and Cathode Material

As described herein, an alluaudite phase of A_(x)T_(y)(PO4)_(z);Na_(x)T_(y)(PO4)_(z); Na_(1.702)Fe₃(PO4)₃ and Na_(0.872)Fe₃(PO4)₃, canbe utilized as new Na-ion battery cathode materials. These alluauditecompositions are described including the hydrothermal synthesis, crystalstructure, and electrochemical properties. In at least some of thealluaudite compositions, A is selected from a group consisting of Li,Na, Mg, Ca and combinations thereof. Additionally, T is selected from agroup consisting of Fe, Mn, Co, Ni, Al, Sn and combinations thereof.After ball milling and carbon coating, the compositions described hereindemonstrate a reversible capacity, such as about 140.7 mAh/g. Inaddition these compositions exhibit good cycling performance (93% of theinitial capacity is retained after 50 cycles) and excellent ratecapability. These alluaudite compounds represent a new cathode materialfor large-scale battery applications that are earth-abundant andsustainable.

Sodium-ion batteries are very promising for large-scale storageapplications. Covalent polyanionic compounds based on earth-abundantmetals have been studied in recent years in the search for new cathodematerials for Na-ion batteries. As described herein, a new Na-ionbattery cathode material, an alluaudite phase of A_(x)T_(y)(PO4)_(z),Na_(x)T_(y)(PO4)_(z), Na_(1.702)Fe₃(PO4)₃ and Na_(0.872)Fe₃(PO4)₃, isdescribed including the hydrothermal synthesis, crystal structure, andelectrochemical properties. Among these compounds, alluaudite phaseswith the chemical formula, Na_(x)T_(y)(PO4)_(z), where T sites areoccupied by Fe, Mn, Co, Ni, Al, Sn and combinations thereof, areadvantageous in their specific embodiments because of their channeledstructures, high theoretical capacity (160 mAh/g), and good thermalstability. Specifically, the alluaudite Na_(1.702)Fe₃(PO₄)₃, andNa_(0.872)Fe₃(PO₄)₃, as described in more detail below, exhibit goodthermal stability and capacity when made via a hydrothermal synthesis.

In one example embodiment, an energy storage composition comprises theformula: A_(x)T_(y)(PO₄)_(z). In at least one example embodiment, theenergy storage composition of where A is selected from a groupconsisting of Li, Na, Mg, Ca and combinations thereof. In relatedembodiments, T is selected from a group consisting of Fe, Mn, Co, Ni,Al, Sn and combinations thereof. It should be appreciated that incertain embodiments of the energy storage composition, x is a numbergreater than or equal to 0 and less than or equal to 3. In other relatedembodiments, y is at least 3 or greater than or equal to 1 and less thanor equal to 3.5 and z is greater than or equal to 1 and less than orequal to 3. Optionally, energy storage composition is coated withcarbon, a carbon-based material and combinations thereof. In embodimentsthat utilize a carbon based material, these materials are selected froma group consisting of polymers, graphite powders, oligomers, graphenesheets, citric acid, ascorbic acid, glucose, sucrose, cellulose,carbohydrates and combinations thereof. In other related embodiments,the maximum capacity of the composition is about 160 mAh/g; delivers areversible capacity of about 46 mAh/g to about 65 mAh/g or 65 mAh/g toabout 100 mAh/g or about 120 mAh/g to about 160 mAh/g.

In one example embodiment, an energy storage composition comprises theformula: Na_(x)T_(y)(PO₄)_(z). In related embodiments, T is selectedfrom a group consisting of Fe, Mn, Co, Ni, Al, Sn and combinationsthereof. It should be appreciated that in certain embodiments of theenergy storage composition, x is a number greater than or equal to 0 andless than or equal to 3. In other related embodiments x is a numbergreater than or equal to 0.872 and less than or equal to 1.702. In otherrelated embodiments, y is at least 3 or greater than or equal to 1 andless than or equal to 3.5 and z is greater than or equal to 1 and lessthan or equal to 3. Optionally, energy storage composition is coatedwith carbon, a carbon-based material and combinations thereof. Inembodiments that utilize a carbon based material, these materials areselected from a group consisting of polymers, graphite powders,oligomers, graphene sheets, citric acid, ascorbic acid, glucose,sucrose, cellulose, carbohydrates and combinations thereof. In otherrelated embodiments the maximum capacity of the composition is about 160mAh/g; delivers a reversible capacity of about 46 mAh/g to about 65mAh/g or 65 mAh/g to about 100 mAh/g or about 120 mAh/g to about 160mAh/g.

In one example embodiment, an energy storage composition comprises theformula: Na_(1.702)Fe_(y)(PO₄)_(z). In other related embodiments, y isat least 3 or greater than or equal to 1 and less than or equal to 3.5and z is greater than or equal to 1 and less than or equal to 3. In atleast one example embodiment, the energy storage composition comprisesthe formula Na_(1.702)Fe₃(PO₄)₃. Optionally, energy storage compositionis coated with carbon, a carbon-based material and combinations thereof.In embodiments that utilize a carbon based material, these materials areselected from a group consisting of polymers, graphite powders,oligomers, graphene sheets, citric acid, ascorbic acid, glucose,sucrose, cellulose, carbohydrates 5 and combinations thereof. In otherrelated embodiments the maximum capacity of the composition is about 160mAh/g; delivers a reversible capacity of about 46 mAh/g to about 65mAh/g or 65 mAh/g to about 100 mAh/g or about 120 mAh/g to about 160mAh/g.

In one example embodiment, a cathode material comprises the formula:Na_(1.702)Fe_(y)(PO₄)_(z). In other related embodiments, y is at least 3or greater than or equal to 1 and less than or equal to 3.5 and z isgreater than or equal to 1 and less than or equal to 3. In at least oneexample embodiment, the cathode material comprises the formulaNa_(1.702)Fe₃(PO₄)₃. Optionally, the cathode material is coated withcarbon, a carbon-based material and combinations thereof. In embodimentsthat utilize a carbon based material, these materials are selected froma group consisting of polymers, graphite powders, oligomers, graphenesheets, citric acid, ascorbic acid, glucose, sucrose, cellulose,carbohydrates and combinations thereof. In other related embodiments themaximum capacity of the composition is about 160 mAh/g; delivers areversible capacity of about 46 mAh/g to about 65 mAh/g or 65 mAh/g toabout 100 mAh/g or about 120 mAh/g to about 160 mAh/g.

In one example embodiment, an energy storage composition comprises theformula: Na_(0.872)Fe_(y)(PO₄)_(z). In other related embodiments, y isat least 3 or greater than or equal to 1 and less than or equal to 3.5and z is greater than or equal to 1 and less than or equal to 3. In atleast one example embodiment, the energy storage composition comprisesthe formula Na_(0.872)Fe₃(PO₄)₃. Optionally, energy storage compositionis coated with carbon, a carbon-based material and combinations thereof.In embodiments that utilize a carbon based material, these materials areselected from a group consisting of polymers, graphite powders,oligomers, graphene sheets, citric acid, ascorbic acid, glucose,sucrose, cellulose, carbohydrates and combinations thereof. In otherrelated embodiments the maximum capacity of the composition is about 160mAh/g; delivers a reversible capacity of about 46 mAh/g to about 65mAh/g or 65 mAh/g to about 100 mAh/g or about 120 mAh/g to about 160mAh/g.

In one example embodiment, a cathode material comprises the formula:Na_(0.872)Fe_(y)(PO₄)_(z). In other related embodiments, y is at least 3or greater than or equal to 1 and less than or equal to 3.5 and z isgreater than or equal to 1 and less than or equal to 3. In at least oneexample embodiment, the cathode material comprises the formulaNa_(0.872)Fe₃(PO₄)₃. Optionally, the cathode material is coated withcarbon, a carbon-based material and combinations thereof. In embodimentsthat utilize a carbon based material, these materials are selected froma group consisting of polymers, graphite powders, oligomers, graphenesheets, citric acid, ascorbic acid, glucose, sucrose, cellulose,carbohydrates and combinations thereof. In other related embodiments themaximum capacity of the composition is about 160 mAh/g; delivers areversible capacity of about 46 mAh/g to about 65 mAh/g or 65 mAh/g toabout 100 mAh/g or about 120 mAh/g to about 160 mAh/g.

FIG. 4a and FIG. 4b illustrate one example embodiment of energy storagecomposition. In at least this example embodiment, the alluauditecomposition Na_(1.702)Fe₃(PO₄)₃ crystallizes in the monoclinic C 2/cspace group. The crystal structure of Na_(1.702)Fe₃(PO₄)₃ can bedescribed as a framework consisting of Fe octahedra with bridgingphosphate tetrahedral. In at least this example embodiment thetheoretical capacity of Na_(1.702)Fe₃(PO₄)₃ is 163.3 mAh/g.

FIG. 6 illustrates an energy storage composition comprising analluaudite compound. In one particular embodiment, Na_(1.702)Fe₃(PO₄)₃.In at least this example embodiment, Na_(1.702)Fe₃(PO₄)₃ crystallizes inthe monoclinic C 2/c space group. The X-ray Diffraction (XRD) pattern ofan annealed sample indicates that pure alluaudite Na_(1.702)Fe₃(PO₄)₃was obtained (see FIG. 6a ). As shown in FIG. 6b and Tables 1 and 2,below, data based on the Reitveld refinement of the XRD, the crystalstructure of Na_(1.702)Fe₃(PO₄)₃ is described as a framework consistingof Fe octahedral with bridging phosphate tetrahedral. Two differentchannels are present in the crystal lattice where sodium ions canreside. These channels are parallel to the c-axis.

FIGS. 7a-c , illustrate SEM images of three samples ofNa_(1.702)Fe₃(PO₄)₃. In this particular embodiment, the energy storagecomposition is shown as-synthesized, annealed, and carbon coated,respectively. Hydrothermal reaction provides unique morphologies bycareful control of synthesis parameters, such as concentration, pH andtemperature. The product contains micro plates with a length of about 2μm and a width of about 300 nm. Optionally, annealing can be used toremove defects and impurity. On average, the length and width of thenanoplate is 2 μm and 200 nm.

TABLE 1 Lattice parameters of Na_(1.702)Fe₃(PO₄)₃ obtained by Rietveldrefinement of XRD data. Phase Alluaudite Space C 2/c Lattice Parametersa (Å) 11.87458(48) b (Å) 12.54469(50) c (Å) 6.48458(23) Beta (°)114.7536(28)

The carbon content of the carbon-coated sample is shown having 5 wt % asmeasured by a Carbon-Nitrogen Elemental Analyzer (CE Instruments ModelNC2100). FIG. 3d is a TEM image of an individual nanoplate ofNa_(1.702)Fe₃(PO₄)₃. The crystalline nature of the as-synthesizedNa_(1.702)Fe₃(PO₄)₃ is confirmed by the Selected Area ElectronDiffraction (SAED) pattern (inset), which is indexed to the monocliniccrystal structure along the [010] zone axis. In at least this exampleembodiment, the energy storage composition comprises a b-axis that isperpendicular to the major facet shown, with the a-axis and c-axiscorresponding to the short and long edge of the crystalline nanoplate.FIG. 8 illustrates a larger contact area greater than or equal to theelectrode and electrolyte. Additionally, the energy storage compositiondescribed herein may also have a higher stress tolerance upon ion(de)intercalation.

FIG. 9 illustrates an example embodiment of a carbon coating process.Phosphate based materials typically exhibit low electronic conductivity.In this example embodiment, the carbon coating is used to improve theelectrochemical performance of the cathode material and/or energystorage composition.

TABLE 2 Atomic coordinates and site occupancies of Na_(1.702)Fe₃(PO₄)₃.Site Np x y z Atom Occ Beg Na1 4 0.00000 −0.0095(19) 0.25000 Na+1 0.7541.66(55) Na2 4 0.50000 0.00000 0.00000 Na+1 0.948 0.27(35) Fe1 4 0.000000.26450(57) 0.25000 Fe 1 0.62(19) Fe2 8 0.28423(57)  0.65817(45)0.3689(11) Fe 1 1.63(15) P1 4 0.00000 −0.2890(11) 0.25000 P 1 2.04(37)P2 8 0.23974(97)  −0.11294(72) 0.1376(19) P 1 2.74(31) O1 8 0.4565(19)0.7275(20) 0.5214(39) O−2 1 7.18(88) O2 8 0.1055(15) 0.6419(12)0.2289(27) O−2 1 0.52(44) O3 8 0.3354(19) 0.6587(16) 0.0722(38) O−2 15.42(65) O4 8 0.1249(16) 0.4037(12) 0.3446(29) O−2 1 2.72(54) O5 80.2182(18) 0.8349(16) 0.3505(34) O−2 1 4.23(64) O6 8 0.3386(13)0.5036(19) 0.3868(28) O−2 1 4.47(56)

FIG. 10a illustrates a sample of carbon-coated Na_(1.702)Fe₃(PO₄)₃nanoplates that were used as a cathode material in a coin-cell battery.As graphically shown charging and discharging profiles of a Na-ionbattery [Na metal∥Na_(1.702)Fe₃(PO₄)₃] are graphically illustrated. Thesodium-ion battery delivers a capacity of ˜60 mAh/g when dischargedgalvanostatically at room temperature. FIG. 10b , discloses sixdifferent plateaus in the voltage-capacity curve, which correspond topeaks in the cyclic voltammetry. These peaks appear at six differentpotentials, indicating Na sites at different energy levels are involvedin the electrochemical charging and discharging of Na_(1.702)Fe₃(PO₄)₃.Broad peaks labelled with black arrows in FIG. 10b at 2.53 V, 2.81 V and3.80 V suggest that single-phase reactions take place. Conversely, sharppeaks labelled with black arrows in FIG. 10b at 2.12 V, 2.96 V, and 3.25V are characteristic of two-phase reactions. CV data obtained at higherscan rate (dashed line in FIG. 10b ) reveal broad peaks at 2.53 V, 2.81V and 3.80 V. CV data obtained at a lower scan rate (solid line in FIG.10b ) reveals the sharp peaks at 2.12 V, 2.96 V, and 3.25 V moreclearly. These results indicate that both single-phase reactions andbiphasic transitions occur in Na_(1.702)Fe₃(PO₄)₃ when cycled in aNa-ion battery.

FIG. 11 illustrates the discharging capacity obtained at the 30th cycle(63.1 mAh/g), as shown in FIG. 11, is much lower than the theoreticalcapacity (160 mAh/g). At least in FIG. 11, three Na ions are assumed tobe electroactive per formula unit if the material is cycled between thetwo end states: Fe^(III)(PO₄)₃ and Na₃Fe^(II)(PO₄)₃.

Chemical oxidation of the as-synthesized Na_(1.702)Fe₃(PO₄)₃ wasperformed using nitronium tetrafluoroborate (NO₂BF₄), which is a strongoxidizer (the potential of the NO²⁺/NO₂ redox couple is 4.5 V vs.Na⁺/Na). An XRD pattern of the chemically de-sodiated material wasmeasured and Rietveld refined to reveal another energy storagecomposition in the alluaudite class, Na_(0.872)Fe₃(PO₄)₃. Refinement ofthe occupancy factors of the Na sites indicates that occupancy of Na(1)and Na(2) sites was reduced to 0 and 0.872, respectively (see Tables 3and 4).

A comparison of the structures of Na_(0.872)Fe₃(PO₄)₃ andNa_(1.702)Fe₃(PO₄)₃ reveals that all Na ions (0.754 Na ions per formulaunit) that occupy the Na(1) site in channel 2 are extracted atpotentials <4.5V vs. Na⁺/Na, while only a small portion of Na ions(0.076 Na ions) that occupy the Na(2) site in channel 1 can be extractedat these potentials. This result can be explained by the difference insize of the two channels: channel 1 is slightly smaller than channel 2(the shortest Na—O bond in channel 1 is shorter than in channel 2 byabout 6.3%) and therefore, more energy is required to extract all Naions from the Na(2) sites in channel 1.

The thermal stability of pristine Na_(1.702)Fe₃(PO₄)₃ was tested by insitu temperature-dependent XRD between room temperature and 500° C. Itexhibits excellent thermal stability as indicated by the absence of anychange in the XRD at all temperatures tested (50° C., 100° C., 200° C.,300° C., 400° C. and 500° C.).

TABLE 3 Lattice parameters of Na_(0.872)Fe₃(PO₄)₃. Phase AlluauditeSpace C 2/c Lattice Parameters a (Å) 11.8532(14) b (Å) 12.5054(3) c (Å)116.40155(58) Beta (°) 114.425(12)

TABLE 4 Atomic coordinates and site occupancies of Na_(0.872)Fe₃(PO₄)₃.Site Np x y z Atom Occ Na1 4 0.00000 −0.01170 0.25000 Na+1 0 Na2 40.50000 0.00000 0.00000 Na+1 0.872 Fe1 4 0.00000 0.25793 0.25000 Fe 1Fe2 8 0.26824 0.64630 0.34254 Fe 1 P1 4 0.00000 −0.31044 0.25000 P 1 P28 0.24002 −0.13254 0.15841 P 1 O1 8 0.44369 0.72210 0.49623 O−2 1 O2 80.07479 0.71801 0.13809 O−2 1 O3 8 0.31627 0.69215 0.06666 O−2 1 O4 80.10103 0.37222 0.26679 O−2 1 O5 8 0.24107 0.84124 0.40714 O−2 1 O6 80.32335 0.50200 0.39185 O−2 1

Method of Making

The methods disclosed herein describe a novel synthesis that includesball-milled, carbon-coated and carbon coated/ball milled processes forcreating an energy storage composition for stationary power sources. Themethods described herein also describe a novel synthesis that includesball-milled and carbon-coated processes for creating a cathode materialfor Na-ion batteries. These materials exhibit high reversible capacity,high voltage, excellent rate capability, good cycling characteristics,and thermal stability—characteristics that are necessary for large-scalebatteries based on earth-abundant materials.

In at least one example embodiment, Na_(1.702)Fe₃(PO₄)₃ was prepared viahydrothermal synthesis. FIG. 5 where starting materials were(NH₄)₂Fe(SO₄)₂.6H₂O (Aldrich, St. Louis, Mo.), H₃PO₄ (Fisher Scientific,Waltham, Mass.), and NaOH (Aldrich, St. Louis, Mo.). Reactants weredissolved in water with a 1:1:3 molar ratio and subsequently transferredto a Parr autoclave, which was sealed and heated at 180° C. for 6 hours.After cooling to room temperature, the reaction was filtered ofprecipitated product. In at least this example embodiment, the productwas dried to yield a fine powder with a greenish grey color. In relatedembodiments, an SPEX 8000D MIXER/Mill® (SPEX SamplePrep, Metuchen, N.J.)is optionally used to ball-mill as-synthesized samples ofNa_(1.702)Fe₃(PO₄)₃.

In order to optionally carbon coat an energy storage composition, adried powder of Na_(1.702)Fe₃(PO₄)₃ was added to a small amount ofethanol that contained 80 wt % of citric acid (Aldrich). This mixturewas sonicated to wet the powder completely with citric acid solution andsubsequently heated at 600° C. under Ar for 5 hours to deposit a carboncoating. A control sample was prepared by annealing theNa_(1.702)Fe₃(PO₄)₃ powder in the absence of citric acid at 600° C.under flowing Ar for 5 hours.

In one example embodiment, a method of coating an energy storagecomposition or cathode material comprises the steps of combining a driedpowder of an energy storage composition with a solution and heating themixture of energy storage composition and solution. In relatedembodiments the solution is selected from the group consisting of citricacid, glucose, lithium citrate, sodium citrate, carbon based monomersand carbon based polymers. In related examples the energy storagecomposition is heated at temperature parameters comprising 600° C. orgreater, 500° C. or greater, 400° C. or greater, 300° C. or greater,200° C. or greater, or no greater than 750° C. In other aspects of themethod of coating the energy storage composition is heated for at least1 hour or at least 5 hours. The energy storage composition is exposed toa gas selected from a group consisting of Argon and Nitrogen.

In related methods, the carbon coated energy storage composition isoptionally exposed to ball milling. In embodiments where ball milling isperformed the composition undergoes high energy tumbling of energystorage compound for at least 15 minutes, at least 1 hour, at least 2hours, at least 3 hours or at least 4 hours. In embodiments where ballmilling is employed, at least 90% of the energy storage compositionparticle size is less than 200 nm. In other related embodiments whereball milling is used, at least 99% of the energy storage compositionparticle size is less than 200 nm.

Scanning Electron Microscopy (SEM) using a field emission microscope(LEO 1530) operating at 10 kV was used to characterize the morphology ofall samples. FEI CM 20—Transmission Electron Microscope (TEM) operatingat 200 kV was used for TEM studies. A D8-DISCOVER® (Bruker Corp.,Billerica, Mass.) diffractometer (operating at 40 mA, 40 kV) equippedwith a Cu-Ku radiation source was used to obtain powder X-raydiffractograms.

Active materials were mixed initially with 20 wt % SUPER P® carbon black(Imerys Graphite (formerly Timcal), Bodio, Switzerland) and a 10 wt %solution of polyvinylidene difluoride (PVDF) in N-methyl-2-pyrrolidone(NMP) to form a slurry. After stirring at room temperature overnight,the slurry was skimmed onto aluminum foil using a doctor blade.Subsequently, samples were dried for 6 h at 110° C. under vacuum. Coincell batteries were assembled in an inert atmosphere dry box ([H₂O]<0.1ppm, ([O₂]<0.1 ppm) using thin discs of metallic sodium as the anode, aglass microfiber filter (grade GF/F; Whatman, U.S.) and Celgard 2400 asthe separator, and 1 M NaClO₄ in propylene carbonate as electrolyte. Thebatteries were cycled galvanostatically at room temperature.

FIG. 7d (inset) illustrates a crystal of the as-synthesizedNa_(1.702)Fe₃(PO₄)₃ that was indexed in the TEM diffraction pattern todetermine the orientation of the Na ion channels relative to the crystalfacets. Indexing revealed the longest edge of the crystal corresponds tothe c-axis of Na_(1.702)Fe₃(PO₄)₃ alluaudite structure, which isparallel to the Na ion channels. Consequently, the diffusion length forNa ions during charging and discharging is longer than 1 μm, which couldexplain the poor performance of the material in previous studies.Therefore, material samples are ball-milled to reduce their particlesize and thus obtain better electrochemical performance.

Ball milling is used to grind materials into a fine powder by mechanicalimpact. FIG. 12 illustrates the theory of ball milling. FIG. 13describes one embodiment of energy storage composition. As shown,Na_(1.702)Fe₃(PO₄)₃ is exposed to ball milling, which alters size andmorphology of the composition. FIGS. 14 and 15 illustrate one ballmilled sample delivering a discharge capacity of 126.5 mAh/g. Further,discharge capacity fading was shown for 30 cycles. In at least oneexample embodiment, an method of making an energy storage composition orcathode material comprising the steps of dissolving reactants(NH₄)₂Fe(SO₄)₂.6H₂O, H₃PO₄ and NaOH in water, thereby forming a cathodesolution and heating the cathode solution, thereby forming aprecipitate. The heating step may be performed for at least 2 hours, atleast 3 hours, at least 4 hours and at least 5 hours. The method mayoptionally include performing the heating step under pressure. In atleast these example embodiments, the pressure is at least 30 psi. Inother related embodiments, the pressure is 40 psi. In some embodiments,an autoclave is used.

In related embodiments, the reactants are dissolved in water with a1:1:3 molar ratio. Optionally, the precipitate may be cooled andoptionally be filtered. In embodiments where a precipitate is cooled andfiltered, the energy storage composition or cathode material may befurther dried to form a powder. In at least one example embodiment, thecathode material is Na_(1.702)Fe_(y)(PO₄)_(z). In these exampleembodiments, y is at least 3 or greater than or equal to 1 and less thanor equal to 3.5 and z is greater than or equal to 1 and less than orequal to 3. In at least one example embodiment, the cathode materialcomprises the formula Na_(1.702)Fe₃(PO₄)₃. Optionally, the cathodematerial is coated with carbon, a carbon-based material and combinationsthereof. In embodiments that utilize a carbon based material, thesematerials are selected from a group consisting of polymers, graphitepowders, oligomers, graphene sheets, citric acid, ascorbic acid,glucose, sucrose, cellulose, carbohydrates and combinations thereof. Inother related embodiments the maximum capacity of the composition isabout 160 mAh/g; delivers a reversible capacity of about 46 mAh/g toabout 65 mAh/g or 65 mAh/g to about 100 mAh/g or about 120 mAh/g toabout 160 mAh/g.

FIGS. 16-18 disclose a comparison of an energy storage composition(Na_(1.702)Fe₃(PO₄)₃) that has been ball milled, but is either carboncoated or non-coated. FIG. 16, carbon-coated ball milled sample (solidline) and non-carbon coated ball-milled sample (dashed line) dischargecapacities were compared. The carbon coated ball-milled sample deliversa discharging capacity of ˜140 mAh/g. FIG. 17 illustrates no dischargecapacity fading after 25 cycles for carbon coated, ball milledNa_(1.702)Fe₃(PO₄)₃. FIG. 18a illustrates an SEM micrograph ofNa_(1.702)Fe₃(PO₄)₃ after ball-milling, where most particles are smallerthan 200 nm. FIG. 18b illustrates an SEM micrograph of a ball-milledsample of Na_(1.702)Fe₃(PO₄)₃ after carbon coating. XRD patterns of thesamples after ball-milling determined broader and lower-intensity peaks,which also indicates that the crystallite size becomes smaller after theball-milling process.

Electrochemical studies were performed on both samples as cathodematerials in a Na-ion coin cell. The ball-milled sample shows adischarging capacity of 126.5 mAh/g for the first cycle (see FIG. 19,opened circles), which is improved compared to the non ball-milledsample. However, this cathode exhibits significant capacity fading toonly 45 mAh/g by the 30th cycle (see FIG. 19, opened circles). Althoughnot wanting to be bound by any particular theory, the reduced capacityretention is likely due to the decreased crystallinity and surfacedefect resulted from the high-energy ball milling process. Improvementin battery performance is achieved after carbon coating of theball-milled material.

The ball-milled/carbon-coated material delivers a reversible capacity of140.7 mAh/g at C/20 rate (8.2 mA/g), which is close to the theoreticalcapacity of Na_(1.702)Fe₃(PO₄)₃ (≈160 mAh/g). This capacity is thehighest value reported for a Na-ion battery using an alluaudite-basedcathode. Indeed, this battery exhibits very good cycling performancewith 93% of the initial discharge capacity retained after 50 cycles (seeFIG. 17). In addition, the average voltage of theball-milled/carbon-coated material is much higher than the sample notcarbon-coated (2.88 V vs. 2.48 V). As a result, the energy density forthe ball-milled/carbon-coated Na_(1.702)Fe₃(PO₄)₃ cathode in a Na-ionbattery is very high (405 Wh/kg). This value is close to the value forLiMn₂O₄ (about 430 Wh/kg) and comparable to the value for LiFePO₄ (about500 Wh/kg) in Li-ion batteries. Improved battery performance can beexplained by the increased conductivity and more ordered surface layerof the electrode material after carbon coating.

Fewer plateaus appear in the charging and discharging curves for theball-milled/carbon-coated material compared to the sample that had notbeen ball-milled (see FIG. 10a ). Only two broad peaks and two sharppeaks are present in the CV data (not shown), implying fewer phasetransitions during cycling. The differences in extraction/insertionbehavior between samples can be interpreted as a decrease in themiscibility gap due to reduction of particle size to the nanoscale. Inaddition, the disorder of electrode materials introduced by ball-millingprocess can also affect the phase transition behavior as reported in thecase of nano-sized LiFePO₄ upon cycling. Na_(1.702)Fe₃(PO₄)₃ crystalsthat have been ball-milled and carbon-coated exhibits excellent rateperformance when compared with other known polyanion-type cathodematerials proposed for Na-ion batteries (see FIG. 21). This particularembodiment of an alluaudite compound may perform better thancarbon-coated LiFePO₄ (in a Li-ion battery).

FIG. 20 illustrates rate capability of Na_(1.702)Fe₃(PO₄)₃. The capacityof a rechargeable battery is commonly rated at 1C, meaning that a 1,000mAh battery should provide a current of 1,000 mA for one hour. The samebattery discharging at 0.5C would provide 500 mA for two hours, and at2C, the 1,000 mAh battery would deliver 2,000 mA for 30 minutes. 1C isalso known as a one-hour discharge; a 0.5C is a two-hour, and a 2C is ahalf-hour discharge. As shown, 5C equals ⅕ of an hour, which equates toa 12 minute discharge. FIG. 21 is a graphical comparison ofNa_(1.702)Fe₃(PO₄)₃ versus other cathode materials. The capacity of arechargeable battery is commonly rated at 1C, meaning that a 1,000 mAhbattery should provide a current of 1,000 mA for one hour. The samebattery discharging at 0.5C would provide 500 mA for two hours, and at2C, the 1,000 mAh battery would deliver 2,000 mA for 30 minutes. 1C isalso known as a one-hour discharge; a 0.5C is a two-hour, and a 2C is ahalf-hour discharge.

It should be apparent to one of ordinary skill in the art that thecompositions and methods described herein can be used in severalstationary power source applications. Although not wanting to be boundby any particular theory, a non-exhaustive list of stationary powerstorage configurations include; grid-scale storage, household energystorage, marine vessel energy storage and stationary constructionstorage.

In other example embodiments the energy storage composition (or cathodematerial) are used to make a battery. In at least one exampleembodiment, a battery comprises: a cathode comprising a cathodecomposite layer on a surface of a cathode collector having a cathodeactive material; an anode including an anode active material; aseparator disposed between said cathode and said anode; and anelectrolyte including ions, wherein said cathode active material is anenergy storage composition. In related embodiments, the cathode activematerial is represented by the formula: A_(x)T_(y)(PO₄)_(z). In at leastone related embodiment, the cathode active material contains particleshaving an average particle size of greater than or equal to about 200 nmand less than or equal to 1 μm.

In at least one example embodiment, the energy storage composition whereA is selected from a group consisting of Li, Na, Mg, Ca and combinationsthereof. In related embodiments, T is selected from a group consistingof Fe, Mn, Co, Ni, Al, Sn and combinations thereof. It should beappreciated that in certain embodiments of the energy storagecomposition, x is a number greater than or equal to 0 and less than orequal to 3. In other related embodiments, y is at least 3 or greaterthan or equal to 1 and less than or equal to 3.5 and z is greater thanor equal to 1 and less than or equal to 3. Optionally, energy storagecomposition is coated with carbon, a carbon-based material andcombinations thereof. In embodiments that utilize a carbon basedmaterial, these materials are selected from a group consisting ofpolymers, graphite powders, oligomers, graphene sheets, citric acid,ascorbic acid, glucose, sucrose, cellulose, carbohydrates andcombinations thereof. In other related embodiments the maximum capacityof the composition is about 160 mAh/g; delivers a reversible capacity ofabout 46 mAh/g to about 65 mAh/g or 65 mAh/g to about 100 mAh/g or about120 mAh/g to about 160 mAh/g.

The synthesis, structure, electrochemical properties of the alluauditesA_(x)T_(y)(PO4)_(z), Na_(x)T_(y)(PO4)_(z), Na_(1.702)Fe₃(PO4)₃ andNa_(0.872)Fe₃(PO4)₃ are described herein. As disclosed above, thecomposition phase of Na_(1.702)Fe₃(PO₄)₃ is shown as an energy storagematerial, used as a cathode in a Na-ion battery. This material wassynthesized using a simple hydrothermal reaction at moderatetemperature. With ball milling and carbon coating, Na_(1.702)Fe₃(PO₄)₃exhibits exceptional electrochemical properties based on the Fe³⁺/Fe²⁺redox couple. The partially de-sodiated compound, Na_(0.872)Fe₃(PO₄)₃,is obtained as a new alluaudite compound. Alluaudite materials with theformulas A_(x)T_(y)(PO4)_(z), Na_(x)T_(y)(PO4)_(z), Na_(1.702)Fe₃(PO4)₃and Na_(0.872)Fe₃(PO4)₃ are a very promising cathode material for Na ionbatteries that target large-scale applications because of its scalableand low cost synthesis, environmentally benign composition, highcapacity (140.7 mAh/g), high energy density (405 Wh/kg), excellent ratecapability, and good thermal stability.

Although exemplary embodiments of principles of this disclosure havebeen described in detail above, those skilled in the art will readilyappreciate that many modifications are possible without materiallydeparting from the principles.

1. An energy storage composition comprising the formula: A_(x)T_(y)(PO4)_(z).
 2. The energy storage composition of claim 1, wherein A is selected from a group consisting of Li, Na, Mg, Ca and combinations thereof
 3. The energy storage composition of claim 1, wherein T is selected from a group consisting of Fe, Mn, Co, Ni, Al, Sn and combinations thereof.
 4. The energy storage composition of claim 1, wherein x is a number greater than or equal to 0 and less than or equal to
 3. 5. The energy storage composition of claim 1, wherein y is at least
 3. 6. The energy storage composition of claim 1, wherein y is greater than or equal to 1 and less than or equal to 3.5.
 7. The energy storage composition of claim 1, wherein z is greater than or equal to 1 and less than or equal to
 3. 8. The energy storage composition of claim 1, wherein the composition is coated with carbon, a carbon-based material and combinations thereof.
 9. The energy storage composition of claim 8, wherein the carbon-based material is selected from a group consisting of polymers, graphite powders, oligomers, graphene sheets, citric acid, ascorbic acid, glucose, sucrose, cellulose, carbohydrates and combinations thereof. 10.-16. (canceled)
 17. The energy storage composition of claim 1, wherein A is Na and wherein x is a number greater than or equal to 0.872 and less than or equal to 1.702. 18.-69. (canceled)
 70. A method of making an energy storage composition or cathode material comprising the steps of: a. dissolving reactants (NH₄)₂Fe(SO₄)₂.6H₂O, H₃PO₄ and NaOH in water, thereby forming a cathode solution; and b. heating the cathode solution, thereby forming a precipitate.
 71. The method of claim 70, wherein the reactants are dissolved in water with a 1:1:3 molar ratio. 72.-74. (canceled)
 75. The method of claim 70, wherein the heating step occurs for at least 2 hours. 76.-90. (canceled)
 91. The method of claim 70, wherein the heating step is under pressure of at least 30 psi. 92.-93. (canceled)
 94. A method of coating a composition comprising the steps of: i. combining a dried powder of an energy storage composition, wherein the energy storage composition comprises the formula: A_(x)T_(y)(PO₄)_(z), with a solution and ii. heating the mixture wherein, the composition is an energy storage composition or cathode material. 95.-97. (canceled)
 98. The method of claim 94, wherein the composition is heated at 400° C. or greater. 99.-100. (canceled)
 101. The method of claim 94, wherein the composition is heated at a temperature of no greater than 750° C. 102.-104. (canceled)
 105. The method of claim 94, wherein the method further comprises the step of exposing the carbon coated composition to ball milling. 106.-112. (canceled)
 113. A method of using an energy storage composition, wherein the energy storage composition comprises the formula: A_(x)T_(y)(PO₄)_(z), and wherein the energy storage composition is used in a process comprising a large scale energy storage system, or a stationary power storage configuration. 114.-115. (canceled)
 116. A battery comprising: i. a cathode comprising a cathode composite layer on a surface of a cathode collector having a cathode active material; ii. an anode including an anode active material; iii. a separator disposed between said cathode and said anode; and an electrolyte including ions, wherein said cathode active material is an energy storage composition, wherein the energy storage composition comprises the formula: A_(x)T_(y)(PO₄)_(z). 117.-131. (canceled) 